DESCRIPTION: This proposed study is to evaluate the feasibility of studying women participating in the Dietary Modification (DM) and the calcium/vitamin D supplementation (CaD) components of the WHI clinical trial (excluding those who are also in the HRT component) to test the hypothesis that adoption of a low-fat dietary pattern is associated with reduced risk of proliferative forms of benign breast disease. Additionally, it is proposed that CaD alone, and in combination with low- fat dietary pattern, will also be associated with reduced risk. Also, it is proposed to conduct a nested case-control analysis of risk factors for proliferative forms of benign breast disease using cases identified in the control arms of the DM and CaD components of the trial. The feasibility study will run for 15 months, including 1 month to generate clinic- specific lists of eligible patients and to train clinics in the study methods, 12 months to conduct the review, and 2 months for data analysis. If the proposed study is feasible, the application predicts that it has the potential to provide insight into the etiology of the putative pre- malignant forms of benign breast disease and might identify an avenue for its prevention.